1. Field of the Invention
The present invention relates generally to offshore drilling and production equipment and in particular to motion compensating apparatus for supporting a marine riser of the type extending from offshore well head equipment to a floating platform.
2. Description of the Prior Art
In performing both drilling and production operations on an offshore well, it is necessary to provide a marine riser connection between well head equipment and a surface facility to provide a stable conduit through which a drill string, production fluids and electrical power may be conveyed between the ocean floor and the surface facility. The surface facility may be a tanker, a drill ship, a barge, a floating platform or a platform which is fixed to the ocean floor. The marine riser cannot withstand compression loading and therefore must be supported under tension at the water surface to prevent its collapse. This is easily accomplished when the surface facility is a production platform which is fixed to the ocean floor, but a more difficult problem is presented when the water depth is so great that the surface facility must be floating and hence is not stationary.
Floating surface facilities such as drilling vessels and other apparatus employed in the drilling of oil wells offshore are generally large and very expensive. Thus it is important that the drilling operations of such a vessel continue with as little interruption as possible. In the offshore environment, sea and weather conditions are generally the determining factor as to whether or not drilling or production operations can continue. The equipment utilized is generally designed to permit continuation of operations in as adverse conditions as possible.
The drilling operations in water depths exceeding several hundred feet are generally performed from a floating, semi-submersible platform, or from a drilling ship, which are supported by buoyancy and not from the sea bottom. Riser tensioner systems have been developed for offshore drilling and production activities to compensate for the rise and fall of the floating vessel. Conventional tensioner systems have commonly comprised hydraulic compensating cylinders connected to cables to the riser at symmetrically arranged tie points. Such riser equipment is generally designed to permit continuation of operations during adverse conditions. However, during periods of severe sea conditions it becomes necessary to terminate the drilling or production operations and disconnect the riser from the well head equipment in order to prevent a catastrophic loss of the well, riser and drill string.
According to conventional procedures, at the onset of adverse conditions the drilling operations are terminated and the drill string is pulled in and stored aboard the vessel. Production operations, during adverse conditions, are discontinued. After that operation, the marine riser is disconnected from the well head equipment and is also disassembled and stored until sea conditions permit resumption of operations. The recovery and subsequent deployment of the drill string and riser involve disassembly, storage and re-assembly operations which are expensive and time consuming. Furthermore, there are occasions when weather conditions become so severe that the drilling vessel must be removed from the production or drilling field as quickly as possible to prevent damage or loss of the drill ship. In such circumstances there may not be sufficient time to recover and store the drill string and riser. This situation becomes progressively more serious in deeper waters where unusually long drill strings and risers are employed. Therefore improved methods and apparatus are needed for retrieving and handling marine risers during emergency disconnect situations.
A problem has arisen which is related to the disconnection of the marine riser while it is undergoing tension loading. In a typical arrangement, the upper end of the marine riser is coupled to the floating platform by means of a telescoping slip joint. When the riser is disconnected, the slip joint collapses very rapidly as the riser is lifted upwardly by the tensioner assembly. Under certain circumstances, a violent collision may occur if the slip joint is permitted to completely collapse or retract during disconnection of the riser.
As the search for petroleum resources advances into deeper waters, the riser length and load increases and thus the tension load imposed upon the riser by the tensioner assembly is increased. For example, when drilling in approximately 4,000 feet of water, the tension requirement will be approximately 500 to 600 kips for a conventional marine riser. The terminal velocity imparted to 4,000 feet of riser by a tension load of 500 to 600 kips is sufficient to cause a violent, destructive impact of the slip joint against the vessel support and rotary table if the slip joint is permitted to collapse.
Although some tensioner assemblies have been equipped with velocity limiter apparatus which have performed well when using relatively short risers, such arrangements are not adequate for preventing slip joint collapse caused by the disconnection of the relatively longer and heavier riser which is undergoing substantially larger tension loading. Because of the serious risk of damage to the drilling vessel and related or production drilling equipment and the risk of serious injury to vessel personnel which may be caused by the violent collision of the riser against drilling vessel, it is imperative to provide an improved riser tensioner assembly which is operable to apply a tension load to the marine riser during normal drilling or production operations and which is operable for relieving the tension load to prevent a violent collision of the riser against the drilling vessel during a planned disconnect or an emergency disconnect operation.